Each casing section is installed inside the previously installed section and consequently its external diameter has to be less than the internal diameter of the installed section. Furthermore it is necessary that this annular gap between the internal diameter of the installed section and the external diameter of the next section is sufficient to accommodate the connecting means between the two sections which includes hanging and packing means as well as the additional diameter of the joints between each length of tubing making up each section. The annular gaps between each subsequent casing section determine the size of the first casing section which is required to be sufficiently large to enable all the required subsequent casing sections to be passed through it and installed in the well. The final casing section is of sufficient diameter to carry out all the desired functions in the production zone of the well which may require over 5 different lengths of casing sections. This results in the first casing section being very large in diameter and therefore expensive and requiring a large diameter hole to be drilled out in order to accommodate it. Further more it is necessary due to the large diameter of the upper sections to extend the smaller diameter lower sections all the away to the surface in order that the required pressure resistance is provided. The objective of the invention therefore is to reduce this required diameter of the sections to considerably reduce the overall costs of the well both in terms of the drilling itself and disposable of the drilled material and in terms of the costs of the large diameter sections.
It has been proposed previously to provide lower diameter sections by reducing the annular space as much as possible, for example in U.S. Pat. No. 5,307,886. The problem with such a narrow annulus and with the method of installation disclosed in this patent and used conventionally is that the well fluids displaced by the introduction and lowering of the subsequent casing section into the well have to pass up the annular space to exit the well at the surface. This presents considerable disadvantages due to the very high friction pressure which are required to be overcome in order for the well fluids to pass up the narrow annular space. Consequently even with high hydrostatic pressures the installation time is very slow time due to the time taken for the fluids to pass up the annular space. Additionally the circulation of cement is very problematic because it relies on the displacement of the mud fluids in the well which are difficult to effectively displace all of the mud which causes incomplete cementing.